The present invention relates to determining, in situ, the radio frequency (RF) shielding effectiveness of gaskets installed in equipment and structures.
Various types of gaskets are used between mating surfaces in electronic equipment and in structures to shield the interior of the equipment or structure against external RF radiation and/or to prevent leakage of RF radiation from the interior of the structure or equipment. The shielding effectiveness of a particular gasket can be measured in the controlled environment of a laboratory in a variety of ways. For example, an RF source may be placed inside an enclosure sealed by the gasket under test and an RF field meter on the outside of the enclosure may measure the amount of RF energy transmitted through the joint or mating surfaces containing the gasket. Similarly, a parallel strip line may be placed on each side of a structure containing the gasket with one strip line acting as an RF transmitter and the other acting as an RF detector.
After an RF gasket has been installed in equipment however, there is no simple test which can be run to determine the shielding effectiveness provided by the gasket. The expected effectiveness may be known from laboratory tests run on the type of gasket used for a particular application before the gasket is installed. However, it will be appreciated that the shielding effectiveness of a gasket degrades over the life of the gasket due to compression set, dirt, abuse, humidity, vibration, etc. Laboratory methods of measuring gasket effectiveness are not suitable for making such measurements in the field because access to both sides of the gasketed space and the creation of other necessary laboratory conditions may not be possible.
Since the gasket typically is installed between two mating metallic surfaces, a simple DC resistance measurement between the two surfaces may reflect a complete breakdown of the gasket or some other major problem. However, even when part of the gasket may have lost its shielding effectiveness, there may still exist a low resistance current path around the defective part of the gasket such that a DC resistance measurement will erroneously suggest that the gasket effectiveness is satisfactory.